1. Technical Field of the Invention
The present invention relates to a piezoelectric resonator, a manufacturing method thereof, and a method of adjusting a resonance frequency of such a resonator. More particularly, the present invention relates to a piezoelectric resonator vibrating in a square vibration mode and provided in a ladder type filter, an oscillator, a filter, a discriminator, or other suitable electronic component, and a method of manufacturing and adjusting a resonance frequency of such a resonator.
2. Description of Related Art
A conventional piezoelectric resonator adapted to vibrate in a square vibration mode is shown in FIG. 11.
The electrode structure of the piezoelectric resonator 51 includes a partial electrode 53 provided on a center area of one major surface of a piezoelectric substrate 52 having a square shape, and an electrode 54 provided on an entire surface of the other major surface of the piezoelectric substrate 52.
The resonance of such a piezoelectric resonator 51 is defined by the resonance frequency thereof. Moreover, the attenuation property when being used for a ladder filter or other similar electronic component is determined by the electrostatic capacitance of the piezoelectric resonator disposed therein.
Therefore, when manufacturing a piezoelectric resonator, the electrostatic capacitance or the value of the resonance frequency must be controlled.
Conventionally, the piezoelectric resonator has been manufactured as shown in FIGS. 12A to 12G.
First, electrically-conductive pastes, such as silver paste is coated almost entirely on the two major surfaces of a piezoelectric mother substrate 55. The silver paste is baked to thereby form electrodes 56 and 57 (FIGS. 12A and 12B).
Subsequently, a DC voltage is applied between the first and second major surfaces of the piezoelectric mother substrate 55 via the electrodes 56 and 57 located on the first and second major surfaces of the piezoelectric mother substrate 55. As a result, a polarization process is achieved so as to apply and define a predetermined piezoelectric characteristic of the piezoelectric mother substrate 55 (FIG. 12C).
After this, pattern printing of resist ink 58 is performed on the electrode 56 at the first major surface of the piezoelectric mother substrate 55 to provide a partial electrode pattern, and printing of resist ink 58 is performed on substantially the entire surface of electrode 57 on the second major surface of the piezoelectric mother substrate 55 to provide a partial electrode pattern (FIG. 12D).
Then, the piezoelectric mother substrate 55, which was printed with the resist ink 58, is soaked in etching liquid so as to remove via etching the electrode portion exposed from the resist ink 58 (FIG. 12E).
If the resist ink is removed, the pattern of the electrode 56 will be formed on the first major surface of the piezoelectric mother substrate 55 so as to define a partial electrode pattern, and the electrode 58 will remain over almost the entire second major surface of the piezoelectric mother substrate 55 (FIG. 12F).
Since the electrostatic capacitance of the piezoelectric resonator 51 is determined by the area of the partial electrode 53, the partial electrode 53 of the piezoelectric resonator 51 is designed so that the electrode area thereof provides a desired electrostatic capacitance.
Therefore, in the above process, resist ink 58 must be printed on the electrodes 56 with high pattern accuracy and the electrodes 56 must be accurately patterned via etching so that the area of the electrode pattern (the partial electrodes) is formed precisely according to design for providing an exact desired electrostatic capacitance (FIG. 12D).
Further, if the piezoelectric mother substrate 55 is cut so that each electrode 56 on the first major surface is centered, the partial electrodes 53 are formed on the first major surface at the center of each piezoelectric substrate 52 that is cut, and the surface electrode 54 is formed on the entire second major surface.
As a result, many piezoelectric resonators 51 adapted to vibrate in the square vibration mode as shown in FIG. 11 are manufactured simultaneously.
After manufacturing the piezoelectric resonator 51 as described above, the resonance frequency of the piezoelectric resonator 51 is measured and adjusted in the following way so that a desired resonance frequency can be obtained.
When the resonance frequency is too high, as shown in FIG. 13, the resonance frequency of the piezoelectric resonator 51 is decreased by grinding the periphery of piezoelectric resonator 51 so as to form a plurality of grooves or cuts 59 in the resonator 51.
Conversely, when a resonance frequency is too low, as shown in FIG. 14, the resonance frequency of the piezoelectric resonator 51 is increased by grinding and chamfering the corners of the piezoelectric resonator.
However, since for the above-mentioned piezoelectric resonator, the area of the partial electrode, i.e., the electrostatic capacitance of the piezoelectric resonator, is determined by pattern printing of the resist ink, in order to manufacture many varieties of piezoelectric resonators having different electrostatic capacitances, it is necessary to separately manufacture every piezoelectric resonator for which a different electrostatic capacitance is required.
Moreover, for the piezoelectric resonator manufactured as mentioned above, the periphery of the partial electrode spreads and blurs because of the resist ink bleeding at the time of pattern printing. The partial electrodes change the shape thereof due to distortions in the printing pattern of resist ink which causes degradation of the electric characteristics of the piezoelectric resonator.
Furthermore, because the partial electrode was formed by etching an electrode, using resist ink as an etching mask, the etching liquid corrodes the piezoelectric substrate and as a result, the electrical properties of the piezoelectric resonator are degraded.
Furthermore, even when the electrostatic capacitance of such a piezoelectric resonator is formed such that it deviates from designed specifications, it is difficult to finely tune the electrostatic capacitance later.
Moreover, in order to adjust the resonance frequency of the piezoelectric resonator, the periphery and especially the comers of the piezoelectric resonator must be grinded or cut off which makes the tuning of the resonance frequency difficult and labor-intensive.
Moreover, using the conventional manufacturing method, in order to form a partial electrode and a whole-surface electrode on both major surfaces of a piezoelectric resonator, many difficult processes are required, production time is long, and cost is very high.
Furthermore, after forming many partial electrodes on a large piezoelectric substrate (piezoelectric mother substrate), since the piezoelectric substrate was made by cutting every area including the partial electrode, the electric characteristics of the piezoelectric substrate degrade when the cut position deviated.